If you've got an iMac G5 that won't eject the disk in it's optical drive, there's a few things you can try:

1. Holding down the mouse button while re-booting your system
2. Booting your iMac into open firmware by holding down Apple + Option + O + F while rebooting, then typing "eject cd" at the prompt (then typing "mac-boot" to get your machine to restart).

Sometimes, though, these basics don't work, and if so, you might be hearing the mechanical noise of the optical drive trying to eject the disc, but something seems to be blocking the disc while the drive tries to eject it, which forces it right back into the machine.

If this is the case, try (CAREFULLY!) positioning your iMac on it's side, such that the optical drive slot is facing down (but of course with space underneath it for the disk to fall out).

Then grab a business card, and slide it about 1/4" into the optical drive slot. Hit the eject button on your keyboard, and as soon as you hear the drive trying to eject, gently move the business card back and forth along the length of the drive slot, keeping it about 1/4" inserted at all times. You may have to try this a few times to get it to work, and this is definitely one of the stranger fixes I've discovered, but it's worked every time I've run into this problem. Hope it works for you!